sha1write: make buffer const-correct

We are passed a "void *" and write it out without ever
touching it; let's indicate that by using "const".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

csum-file.c

@@ -11,7 +11,7 @@
 #include "progress.h"
 #include "csum-file.h"
 
-static void flush(struct sha1file *f, void *buf, unsigned int count)
+static void flush(struct sha1file *f, const void *buf, unsigned int count)
 {
 	if (0 <= f->check_fd && count)  {
 		unsigned char check_buffer[8192];
@@ -86,13 +86,13 @@ int sha1close(struct sha1file *f, unsigned char *result, unsigned int flags)
 	return fd;
 }
 
-int sha1write(struct sha1file *f, void *buf, unsigned int count)
+int sha1write(struct sha1file *f, const void *buf, unsigned int count)
 {
 	while (count) {
 		unsigned offset = f->offset;
 		unsigned left = sizeof(f->buffer) - offset;
 		unsigned nr = count > left ? left : count;
-		void *data;
+		const void *data;
 
 		if (f->do_crc)
 			f->crc32 = crc32(f->crc32, buf, nr);

csum-file.h

@@ -34,7 +34,7 @@ extern struct sha1file *sha1fd(int fd, const char *name);
 extern struct sha1file *sha1fd_check(const char *name);
 extern struct sha1file *sha1fd_throughput(int fd, const char *name, struct progress *tp);
 extern int sha1close(struct sha1file *, unsigned char *, unsigned int);
-extern int sha1write(struct sha1file *, void *, unsigned int);
+extern int sha1write(struct sha1file *, const void *, unsigned int);
 extern void sha1flush(struct sha1file *f);
 extern void crc32_begin(struct sha1file *);
 extern uint32_t crc32_end(struct sha1file *);